In the year 2016 measures will be taken to improve the possibilities for fish to migrate between the Wadden Sea and the IJsselmeer. These measures could result in a trend reversal for the glass eel monitoring at the Stevin Sluizen in Den Oever that forms Europes longest lasting data series for the glasseel index. In 2015 and 2016 field data has been and will be collected concerning the distribution of glass eels with and without the measures and infrastructure to improve fish migration in place. This datasets will be used to identify if a trend reversal in the dataset can be expected after 2015. And if that is the case, to determine a correction factor which can be applied to maintain the continuity of the dataset.